The invention relates to a process and apparatus for the removal of acidic gases such as carbon dioxide and hydrogen sulfide from a hydrocarbon gas. More particularly, it relates to a process for removing these acidic gas components and electrolytically converting the resulting aqueous sodium sulfate solution to sodium hydroxide and sulfuric acid products. These products can then be recycled to the appropriate stages in the inventive process for reuse. The sodium hydroxide is recycled to the sodium hydroxide treatment step and the sulfuric acid is recycled to the reaction step. The hydrogen sulfide can be converted to a usable sulfur product.
In the field of petroleum chemistry, it is highly desirable to remove acidic gases such as carbon dioxide and hydrogen sulfide from hydrocarbon gases which contain these materials generally in relatively small quantities. Illustrative of hydrocarbon gases which often contain these impurities are: gases obtained from the cracking of hydrocarbons such as gas oil, naphtha, ethane, etc. Frequently, naturally occuring hydrocarbons, such as natural gas, contain these undesirable materials. Their undesirablility derives in part from the fact that their acidic nature is prominent under aqueous conditions. This acidic quality can also be detrimental when using the particular gas for its intended use. For instance, they impart corrosive properties and, in the case of hydrogen sulfide, toxic properties. Also, in many instances the presence of these acid gases in hydrocarbon streams is detrimental to subsequent processing of these streams and to the manufacture of chemicals therefrom.
The removal of these unwanted components from a hydrocarbon gas is generally accomplished by scrubbing the gas in a suitable tower with a water solution of a reagent capable of reacting with the acidic gases. Potassium carbonate is frequently used. For example, it reacts with carbon dioxide to form potassium bicarbonate in solution, which may be regenerated by driving off the excess carbon dioxide and either venting it or collecting it for some other usage. However, potassium carbonate is effective for the removal of hydrogen sulfide only in special cases.
Among others, one type of solution frequently employed for scrubbing the gases is a solution containing monoethanolamine. Such a solution can also be regenerated. However, it has three principal drawbacks: relatively high vapor pressure; the fact that it reacts essentially irreversibly with carbonyl sulfide (a common constituent of gas from cracking operations) with resultant substantial loss of the amine; and that it is generally impracticable to remove substantially all the acidic gases using this solvent.
Still another solution which has been used effectively is aqueous sodium hydroxide. This reagent is capable of removing substantially all the acid constituents from gases such as cracked gases and natural gases. Use of this solution, however, suffers from the notable disadvantages that the expensive sodium hydroxide is consumed, it is not recovered, and moreover the waste products from the scrubbing system constitute a potentially serious source of water pollution.
Various removal systems have been devised which are capable of removing these gaseous constituents to various final levels in the treated gases. However, no method has been devised until the present time for substantially complete removal of hydrogen sulfide and/or carbon dioxide from a hydrocarbon gas whereby there is simultaneous stripping of said gases and conversion of H.sub.2 S to useful sulfur with efficient andd economic regeneration of reagent materials in condition suitable for recycling purposes. It is the general purpose of the present invention to provide a process and apparatus for attaining these ends.